NASA scientists rethink the "habitable zone"

Do we share our solar system with other forms of life? Until scientific consensus emerged, in the 1990s, of the existence of vast oceans of liquid water on two of Saturn’s moons and one of Jupiter’s, the question was hard to take seriously. But now, according to panelists at a session at the 2018 South by Southwest Interactive conference, evidence is mounting that when we finally find evidence of extraterrestrial life it might be in our own tiny neighborhood of the galaxy.

A series of stunning new scientific findings are sparking renewed excitement over the possibility that life exists in the vast oceans covering Europa, one of Jupiter’s 67 known moons, or in those on Enceladus or Titan, which both orbit Saturn.

“When we think about the solar system, we’re used to a very traditional picture where we have eight planets,” says Cynthia Phillips, Europa Staff Scientist at NASA’s Jet Propulsion Laboratory (JPL). “For a long time, people have been interested in a thing called the habitable zone, where water can be a liquid on the surface…Only recently have we started to expand our notion of a habitable zone to include the outer planets,” she notes.

Europa, for example, has “all the factors necessary for life,” Phillips asserts. Under an ice crust estimated to be 19 to 25 kilometers thick, Europa has an ocean that contains more liquid water than all the oceans on Earth combined. The moon also has abundant carbon, phosphorous, and nitrogen, she adds. There may also be hydrothermal vents or a “radiation environment” at the surface “that creates molecular species that trickle down into the ocean,” helping to support life. Finally, there has been plenty of time for life to arise and evolve: Europa’s “ocean has been liquid throughout the age of the solar system,” about 4.57 billion years, Phillips says.

To learn more about the mysterious moon, NASA plans to launch a mission called Europa Clipper in the mid-2020s. Because of the high levels of radiation around Europa, the probe won’t go into orbit around the Jovian moon. Rather, it will pass close enough to sample bits of the surface that are kicked into space by the bombardment of micrometeorites that constantly assail the moon’s icy surface. By studying the composition of these bits, as well as images and spectra of the surface, scientists may find “clues of life, if there is life in the ocean,” says Morgan Cable, an astrochemist at NASA’s JPL.

Intense radiation at the surface of Europa could play a key role in sustaining microbial life in the vast ocean, Cable explains. The complex process starts with Jupiter’s enormous magnetic field, which accelerates and sprays solar wind particles at Europa, showering the moon with radiation. At the surface, the particles rip water molecules apart, creating H3O+ and O- ions, among others.

The electron-starved H3O+ ions may sink down into the ocean. Meanwhile, hydrothermal vents on the ocean floor release countless chemical species that are electron rich. The difference between electron deficiency on the surface and excess electrons below may set up a chemical condition known as a “redox (reduction-oxidation) gradient,” Cable notes.

“Many types of life need that gradient to live,” she explains. For example, “sulfate-reducing and sulfate-oxidizing bacteria.” These and other microbes “help move electrons from reduced places to oxidized places, and, in doing so, get energy.”

The new thinking about microbial life comes from increasing alliances among scientists with different specialties, Cable adds. “Geologists are talking to chemists, who are talking to biologists, oceanographers, and people who study hydrothermal vents. A lot of us are starting to attend the same meetings, and talk more, so a lot of the information that was previously isolated in one field is now being disseminated among all of planetary science. These communities have really started coming together.”

Image: JPL-Caltech/NASA

This cutaway view of Saturn moon Enceladus is an artist rendering based on published results from NASA’s Cassini mission. It depicts possible hydrothermal activity that may be taking place on and under the seafloor of the moon’s subsurface ocean.

Cable is particularly excited by a remarkable series of findings that add credence to the belief that seafloor hydrothermal events exist on Saturn’s moon Enceladus. The existence of such vents would greatly strengthen the case for life on the distant moon. On Earth, such vents are known to harbor countless unique species, ranging from bacterium to crabs and octopuses. “These are great oases of life,” says Cable. “We find complete ecosystems, and they’re thriving, as far as we can tell, independently of sunlight and other elements of life” such as oxygen and carbon dioxide.

The case for hydrothermal vents on Enceladus, set out in a paper published last year in Nature by J. Hunter Waite and collaborators, is based on a couple of findings from the Cassini mission. One was the detection of hydrogen in plumes emitted from the moon. “There was so much hydrogen that the only logical explanation is hydrothermal activity,” through a geochemical process called serpentinization, Cable explains.

Also, tiny spheres of silica, called nanograins, were detected by the Cassini probe in 2015. According to Cable, “The only way you can get these is if you have liquid water in contact with the crust of the seafloor at 90 degrees C or above.”

Of the three watery moons, Titan might be the most bizarre of all, Cable adds. Titan, the second biggest moon in the solar system, appears to have a coating of organic molecules, covering a crust of ice, above a liquid-water ocean. It has an atmosphere that is mostly nitrogen and so thick that “if you had wings, and you flapped those wings, you could fly,” Cable points out.

The Cassini spacecraft and Huygens probe found dunes, river valleys, and lakes of liquid methane and ethane on Titan. Says Cable: “As a chemist, that really fascinates me, because any life we might find in these liquid bodies would be very, very different from the life we know on Earth.”

For one thing, she notes, it could not be based on the DNA that encodes the countless traits of creatures on Earth. DNA can dissolve in water because both are electrically polar molecules. However, methane and ethane are both nonpolar, so any analogous molecule for methane-based creatures would have to be nonpolar and, presumably, very large. A few researchers, notably Steven A. Benner of the Westheimer Institute of Science and Technology, have begun trying to conceive of such a molecule, but it has proven elusive.

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